1. Field of the Invention
The present invention generally relates to systems and methods for high fidelity sound reproduction, and more particularly, to systems and methods for separating and directing signals in different frequency bands in a multi-driver, multi-speaker, vehicular audio system.
2. Description of Related Technology
Car stereo systems face unique problems in high fidelity reproduction of recorded or broadcast sound, because speaker placement, speaker types, amplifier power, crossover networks, limited internal space, internal vehicle geometry, and other factors can all affect the quality and characteristics which the listener hears. Increasing amplifier power, despite the consequent expense, does not confront the major problems, which derive both from the limited space available for installations and the complex nature of internal reflections within a vehicle. Acoustic waves launched from a given speaker location into the interior of a vehicle are reflected with relatively short distances off interior surfaces. They then will often reflect back and forth between opposed surfaces to establish standing waves, thus creating resonance peaks within the audible frequency spectrum. Because the interior dimensions of a vehicle are limited, resonances arise in the longer wave (low frequency) region of approximately 60 Hz or less to 300 Hz or more. Moreover, such simple resonances are often accompanied by complex standing waves which are created because of multiple, oblique-angled reflections off different surfaces within the three-dimensional volume of the vehicle.
There has been an increasing recent trend toward improving the fidelity of car audio systems, as opposed to earlier tendencies to use excessive power at low frequency levels. An earlier stereo installation might have used two speakers, each comprising a mid-range and tweeter unit, spaced apart in the front or rear of the vehicle. These would be driven through a crossover network from a single amplifier. It is now common to use "multi-amp" installations, in which speakers for the different frequency ranges are each driven by a separate amplifier. The value of cleaner low frequency ranges has become more apparent and separately driven woofers and sub-woofers are thus increasingly being used. The multi-amp installations include so-called "bi-amps", employing a two-way division of the frequency ranges, and "tri-amps" in which the division is between low frequency (woofer), mid-range unit and high frequency (tweeter). A sub-woofer is alternatively used for the lowest frequency range to enhance bass response, the sub-woofer unit often being monaural.
There are two basic methods of mounting sub-woofers in a vehicle, namely "free air" or in an enclosure (sealed or vented). Mounting sub-woofers in an enclosure that is properly matched to the characteristics of the woofers results in tighter base, better transient frequency response, and higher power handling ability. Unfortunately, all woofers have different characteristics and therefore require custom designed enclosures for maximum performance. To build a proper matching enclosure (commonly known as a tuned enclosure) requires knowledge of complex mathematical equations developed through the science of acoustics.
In brief, mounting any woofer in a sealed enclosure causes the woofers frequency response to roll off below the resonant frequency of the enclosure at a rate of 12 dB per octave. The smaller the enclosure, the higher the resonant frequency becomes. Based on mathematical calculations, a typical woofer requires a sealed enclosure of about 2.2 cubic feet or 1.6 cubic feet if the enclosure is vented. There is rarely available space in an automobile for such a large enclosure. Most vehicles can only accommodate an enclosure of approximately 1.2 cubic feet or less.
The limited space availability in an automobile often lends itself to the placement of a sub-woofer system in the rear and the midrange/tweeter systems in the front and/or along the sides. Due to this placement problem, the acoustical time delay may occur between the time the sub-woofer system is heard as oppose to when the midrange/tweeter system is heard. Furthermore, the limited space in an automobile may allow room for a single sub-woofer installation only. The internal geometry of the vehicle varies with car style, even in a particular model (e.g. two door vs. four door) and with the interior materials that are used. Thus if electronic crossovers are to be used to flatten frequency response, or shape frequency response to the likes of the listener, a design having novel versatility is required.
To achieve a substantially flat frequency response within a vehicle using a multi-amp system, the trend has been to use electronic crossovers. The electronic crossovers are adjustable as to crossover point, and operate more efficiently than do passive crossover networks. Because they are adjustable, a troublesome resonance or null in a given frequency range can be compensated by spacing crossover points so as to diminish response, or overlapping the crossover points so as to enhance response.
Known electronic crossover systems are limited in their capabilities, as presently implemented, because they are generally restricted to two separate independently adjustable frequency bands. It is recognized that they can be cascaded (used in series) to give tri-amp as well as bi-amp capability, but this limits the capability for adjustment because a later crossover can only choose a higher high-pass (or a lower low-pass) level for cutoff.
Additionally, a good audio system should give the listener the feeling of three-dimensional music reproduction with a well defined image of the placement of various instruments, the vocal source across the sound stage, and a distinct impression of good depth of feel. If a system is designed and installed properly, the point location of speaker elements should not be easily identifiable. Instead, it will give one a feeling that he is surrounded by a live orchestra in the concert hall.